Power devices are widely used to carry large currents and support high voltages. Modern power devices are generally fabricated from monocrystalline silicon semiconductor material. One type of power device is the thyristor. A thyristor is a bistable power semiconductor device that can be switched from a nonconducting “off” state to a conducting “on” state, or vice versa. Power semiconductor devices, such as thyristors, high-power bipolar junction transistors (“HPBJT”), or power metal oxide semiconductor field effect transistors (“MOSFET”), are semiconductor devices capable of controlling or passing large amounts of current and blocking high voltages.
Thyristors are generally known and conventionally have three terminals: an anode, a cathode, and a gate. A thyristor may be activated to conduct current between its anode and cathode terminals when a current pulse is received at its gate terminal. More particularly, a thyristor may be turned on by applying a short current pulse across the gate and the cathode. Once the thyristor turns on, the gate may lose its control to turn off the device. The turn off may be achieved by applying a reverse voltage across the anode and the cathode. A specially designed gate turn-off thyristor (“GTO”), however, is typically turned off by a reverse gate pulse. The GTO thyristors generally start conduction by some trigger input and then behave as diodes thereafter.
A thyristor is a highly rugged device in terms of transient currents, di/dt and dv/dt capability. The forward voltage (VF) drop in conventional silicon thyristors is about 1.5 V to 2 V, and for some higher power devices, about 3 V. Therefore, the thyristor can control or pass large amounts of current and effectively block high voltages (i.e., a voltage switch).
Two parameters of a thyristor are the built-in potential (which is a characteristic of the bandgap of the semiconductor material used to manufacture the thyristor) and the specific on-resistance (i.e., the electrical resistance across the anode and cathode of the device in the linear region when the device is turned on). The specific on-resistance for a thyristor is typically as small as possible so as to provide a large current per unit area for a given voltage applied to the thyristor. The lower the specific on-resistance, the lower the forward voltage (VF) drop is for a given current rating. The minimum VF for a given semiconductor material is its built-in potential (voltage).
Some conventional thyristors may be manufactured in silicon (Si) or gallium arsenide (GaAs), such as a silicon controlled rectifier (“SCR”). Thyristors formed in Si or GaAs, however, may have certain performance limitations resulting from the Si or GaAs material itself, such as the minority carrier lifetime and the thickness of the drift region. The largest contributory factor to specific on-resistance is the resistance of the thick low-doped drift region of the thyristor. In a majority carrier device, such as a MOSFET, the specific on-resistance is determined by the doping concentration and the thickness of the lightly doped drift layer. In a minority carrier (or bipolar) device, carriers, both electrons and holes, are injected into this drift layer, and substantially reduces the specific on-resistance. This effect is referred to as conductivity modulation. As the rated voltage of a thyristor increases, typically the thickness of the drift region increases and the doping of the drift region decreases. For effective conductivity modulation, a very long minority carrier lifetime is required. At the same time, the amount of carriers stored in the drift layer increases because the volume of the drift layer is increased. Therefore, the time required to remove access carriers in the drift layer, which determines the switching times and frequencies, may increase dramatically for devices with higher blocking voltage ratings.
Development efforts in power devices have included the use of silicon carbide (SiC) devices for power devices. Silicon carbide has a wide bandgap, a lower dielectric constant, a high breakdown field strength, a high thermal conductivity, and a high saturation electron drift velocity relative to silicon. These characteristics may allow silicon carbide power devices to operate at higher temperatures, higher power levels and with lower specific on-resistance and higher switching frequency than conventional silicon-based power devices. A theoretical analysis of the superiority of silicon carbide devices over silicon devices is found in a publication by Bhatnagar et al. entitled “Comparison of 6H-SiC, 3C-SiC and Si for Power Devices”, TREE Transactions on Electron Devices, Vol. 40, 1993, pp. 645-655.